Effects of Precipitation and Air Temperature on Phosphorus Fluxes from Rhode River Watersheds

Abstract

We studied fluxes of total P, total phosphate, and total organic P from seven contiguous small watersheds on the Atlantic Coastal Plain in Maryland for up to 25 yr. These watersheds have perched aquifers so all groundwater discharges as weD as surface runoff were measured at V -notch weirs equipped with volume-integrating, Dow-proportional samplers. Interannual variations in annual and seasonal precipitation during this study spanned approximately the range of 160-yr weather records in the region. Annual totai-P area yields from the overall watershed varied 28-fold, correlations of all P-species Ouxes with precipitation were highly significant, and power function regressions of precipitation vs. P-flux explained from 42 to 55% of the variance in the latter. Phosphorus Ouxes from a cropland watershed were much higher and more variable with volume of precipitation, while fluxes from a forested watershed were much lower and primarily composed of organic P. Correlations of P fluxes with precipitation were higher in the spring. Annual and seasonal P concentrations also often increased significantly with precipitation. Variations in seasonal mean air temperature sometimes explained significant amounts of variance in P fluxes, especially phosphate from cropland. A regression model was used to construct graphical and tabular summaries.A LARGE number of studies have measured the fluxes ~of phosphate, organic P, and total P from various watersheds. The usual foci have either been comparative effects of land use in a small region (e.g., Cooper and Thomsen, 1988;Correll et al., 1977;Hill, 1981;Hirose and Kuramoto, 1981;Maimer and Grip, 1994), differences in discharges among widely separated regions (e.g., Beaulac and Reckhow, 1982;Dillon and Kirchner, 1975;Frink, 1991), or patterns of change in P concentrations from a single watershed due to a storm event (e.g., Johnson et al., 1976;McDiffett et al., 1989;Prairie and Kalff, 1988). These studies have clearly established that land use, particularly intensive agriculture, has a strong influence on P flux. They have also shown that P fluxes from a given watershed are seasonal, peaking in the spring and summer in temperate regions. The evidence seems fairly good that there are also significant regional differences in P fluxes from a given land use, perhaps due to geological and climatic differences. It is also clear that during individual storm events concentrations of parameters such as total phosphate and total organic P span a broad range and usually reach maxima before the peaks of storm hydrographs. Very few studies have analyzed the effects of interannual variations in precipitation and temperature on fluxes of P species. Such an analysis requires many years of data from the same watershed. For up to 25 yr we have sampled the fluxes of total phosphate, total organic P, and total P from seven contiguous small subwatersheds of the Rhode River in Maryland. The subwatersheds differed in land use, but had similar weather, soils, geology, and hydrology. The watersheds we...

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